Epoxy resins are characterized as one having various excellent properties, such as chemical proof, corrosion resistance, mechanical property, thermal property, adhesive property to various materials, electric property, and easy handling property under any condition, and are widely used for adhesives, paints, electrometal materials and complex materials. An epoxy group in an epoxy resin is a functional group which has great distortion therein and enormous reactivity, being reactive to both acids and bases, and is capable of curing epoxy resins by virtue of such high reactivity to make a resin into three dimension structure. An epoxy resin composition is composed of an epoxy prepolymer, which contains more than 2 epoxy groups in a molecule, and a curative, and is normally added with a curing accelerator, a denaturant, a filler, etc. depending upon the use thereof. It is known that the property of a cured-resin is subject to the type of a curative used, and various curatives have ever been used for industrial purposes. Epoxy resin compositions can be divided into two main types, the one is one-pack mixture and the other is two-pack mixture type, the former type can be cured, for example, by heating, pressing or allowing the composition itself to stand. The other type, two-pack mixture type, can be cured by admixing the main component and either a curative or a curing accelerator just before use and subsequently heating, pressing or allowing the mixture to stand, for example. Normally, the epoxy resin compositions are prepared into two-pack mixture-types, which are widely used for parts to be used in the fields of electric appliances industry, automobile industry and aircrafts industry, since two-pack mixture type has excellent properties in terms of the strength of cured-products, thermal property, electricity property, etc., though it is not easy to handle and not economical from operation point of view. However, the two-pack mixture type has problems that, (1) since it has short pot life, that means time maintainable the state of prepared composition to be usable for curing, operational performance is ceased due to starting of partial curing of the composition during the preparation, which causes the increase of viscosity of the composition, and (2) the physicochemical property of the composition is ceased by incorrect mixing or incomplete preparation. Therefore, latent-type curatives and curing accelerators, which are prepared as one-pack mixture type, have been desired. Latent-type curatives and curing accelerators are defined as ones, in which a curative and a curing accelerator compounded in a resin are stable at room temperature, and which may induce a curing reaction by virtue of an effect such as heating. For the initiation of curing reactions, heat, light, pressure, etc. may be effective, however, it is rather normal to use heating. For stabilizing the effect of curatives and curing accelerators, microcapsules thereof have been used, however, such microcapsules do not have sufficient mechanical strength, and therefore, there have been a problem in stability of those microcapsules such that they cannot stand for a process of blending to adjust resin compositions.
It is known that there are several types of curatives, for example, (1) addition-type curatives, the molecules of a curative are always incorporated into a cured-resin by virtue of the reaction with epoxy groups, (2) polymerization-type curatives, of which molecules enzymatically induce opening of rings of epoxy groups without causing incorporation of molecules of the curative into resins to cause polymerization and addition reaction between oligomers, and (3) photoinitiation-type curatives, which initiate curing by gaining irradiation of ultraviolet rays. Irrespective of the type as described above, it is the most important thing to carry on polymerization addition reaction under a fixed condition and more homogeneously and faster in order to obtain a cured-product in the stable state. However, we have still problems when using any of existing curatives such that (1) curing reaction by using any of existing curatives stops before the completion of the reaction due to increase of viscosity of resins, (2) there are many inhibitory factors against a curing reaction, (3) some severe conditions are required for completing a curing reaction, and (4) a great amount of a curative is required for carrying out a curing reaction homogeneously, and therefore, curing accelerators which enable to proceed homogeneous and fast polymerization addition reaction under a mild condition have badly been required. The curing accelerator is defined here as the one which makes the curing time of a curative for curing epoxy resins shorter and makes the curing reaction faster and more smooth. For addition-type curatives, such as primary amines and secondary amines, alcohols or phenols are used as a curing accelerator for promoting a polymerization-addition reaction. However, there is yet a problem in those use in general, since in case of using any of polymerization-type curatives, such as imidazoles, anion polymerization to be developed between oligomers tends to be inhibited by such alcohols and phenols.
In Japanese Patent Laid-open No. Hei 5-194711 Gazette, an epoxy resin which is compounded with a clathrate comprising both a curative for epoxy resins and a curing accelerator for epoxy resins with a multimolecular (phenol) host compound is described. Specifically, a method for curing epoxy resins by adding a clathrate comprising 2-ethyl-4-methylimidazole and 2,2′-methylenebis(4-methyl-6-t-butylphenol) at a ratio of 1:1 is added into an epoxy resin at a rate of several % based on imidazole is described in the Gazette.
However, although there is a description that a pot life (stability as a one-pack mixture) of the compounded-epoxy resin described above can be prolonged sharply, it is just a comparison with a similar clathrate, cyclodextrin, and the performance of that compounded-epoxy resin is not yet satisfactory for the use in a practical scale. Also, there is no description on the thermal stability and the curability at low temperature in the Gazette.
In Japanese Patent Laid-open No. Hei 5-201902 Gazette, there is a description of a clathrate comprising a tetrakis-phenol compound and an imidazole compound, although there is no definite description that the clathrate can be used as a curative and a curing accelerator for epoxy resins.
In Japanese Patent Laid-open Nos. Sho 60-40125 and Hei 8-151429 Gazettes, an use of a salt of an imidazoline compound and a polyhydric phenol as a curative for epoxy resins is described. However, such curative is neither crystalline solid nor a clathrate and does not give sufficient stabilizing effect as a one-pack mixture, in practice. Again, in U.S. Pat. No. 3,519,576, an use of a salt of an amine compound and a polyhydric phenol compound as a curative for epoxy resins is described. However, this salt is not practically satisfactory as a curative in the light of the stability as a one-pack mixture. In U.S. Pat. No. 4,845,234, an use of a salt of an imidazole compound and a polyhydric phenol compound as a curative for epoxy resins is described. However, the state of this salt is highly-viscous liquid and is not a clathrate compound, and it is not the one which can be practically used with satisfaction in the light of the stability as a one-pack mixture.
In Japanese Patent Publication No. Hei 6-9868, a disclosure of the use of a salt of a tetrakisphenol compound and an imidazole compound as a curative for epoxy resins is made, however, there is no definite description about the use. In this publication, a salt of an imidazole compound and a polyhydric phenol compound is disclosed, however, the state of the salt is highly-viscous liquid and is not formed as a clathrate compound. Though there is a description as to the stability as a one-pack mixture, etc., such effect seems to be practically unsatisfactory. Furthermore, the description lacks an explanation on the heat stability and the curability at low temperatures.
In Japanese Patent Publication No. 2501154 and Japanese Patent Publication No. Hei 7-74260, there is a description that a tetrakisphenol skeleton be introduced into a produced-resin by using a tetrakisphenol compound as a curative. In this case, There is a characteristic in the resin that a tetrakisphenol skeleton be introduced into the produced-resin, where the tetrakisphenol compound as a curative is used at a greater rate of 0.5-2 moles against 1 mole of epoxy groups. As the effect, only description on the stability as a one-pack mixture is given, whereas no description is given about the stability on heat and the curability at low temperatures.
Considering such background as described above, it is an object of the present invention to provide a curative for epoxy resins and a curing accelerator for epoxy resins, which have improved subliming property and decomposing property, remarkably-improved thermal stability which is extremely important for the control of a curing reaction, a prolonged pot life (stability as a one-pack mixture comprising an epoxy resin and a curative) and improved curability at low temperatures. Furthermore, the present invention is also directed to provide an epoxy resin composition which provides stable cured-products even under a mild condition by proceeding a curing reaction of an epoxy resin faster and smoothly, etc.